build: Remove SIZEOF___INT64 definition from meson.build
build: Remove SIZEOF___INT64 definition from meson.build

This was a leftover from use of the __int64 type, which was removed
recently in commit 7e821441c482917e54435a07893272d87d3ad9e5.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

https://gitlab.gnome.org/GNOME/glib/issues/1313

endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib